Weight reduction of a body of an automobile using a high-strength steel sheet has been put forward, in order to suppress an emission amount of carbon dioxide gas from an automobile. A high-strength steel sheet has come to be often used for a body in order also to secure safety of a passenger. Further improvement of strength is important to further proceed with weight reduction of a body. On the other hand, some parts of a body require excellent formability. For example, an excellent hole expandability is required for a high-strength steel sheet for an underbody part.
However, attaining both of a strength improvement and a formability improvement is difficult. In general, the higher a strength of a steel sheet is, the lower a formability is, and an elongation, which is important in drawing and bulging, and a hole expandability, which is important in burring, are reduced.
Patent Literatures 1 to 11 describe high-strength steel sheets intended to improve formability or something. However, a hot-rolled steel sheet having a sufficient strength and a sufficient formability cannot be obtained by the conventional techniques.
Though a technique related to improvement of a hole expandability is described in Non Patent Literature 1, a hot-rolled steel sheet having a sufficient strength and a sufficient formability cannot be obtained by this conventional technique. Further, this conventional technique is hard to be applied to a manufacturing process on an industrial scale of a hot-rolled steel sheet.